Determination of fuel loads is carried out before take-off based on information about the flight plan and the forecast weather situation along the route. More precisely, the statutory loads comprise:                the fuel necessary for reaching the destination, depending on the flight profile and the estimated average wind over the route;        a reserve of 5% for the flight contingencies;        a 30 minute reserve for carrying out a possible holding pattern on arrival before landing; and        a reserve for being rerouted to an airport other than the destination.        
When filling up with fuel, an excess in relation to the statutory loads may be loaded, this excess is commonly called “extra fuel” in the literature.
Generally, the excess is determined before take-off in order to fill up the aircraft with fuel. This calculation is not repeated subsequently. In the case of changes in weather conditions and in particular if the present conditions on the route lead to an increase in fuel consumption, it is possible to arrive at a situation where the excess becomes negative, which is problematic for flight safety.